In effort to get backlinks people used to spam lot of blogs and forums with the links to their site and Google and other search engines came out with a solution to have a nofollow attribute to links which means its not to be given any importance when it comes to SEO.

i m a professional freelancer but i still don't know what is the benifit of backlinking to dofollow blog and comments on forum.can anyone tell me what is the benefit of giving backlink ???

Hi,
The benefits of having backlinks are many – the more links a website has coming into it, the more authoritive and popular it looks to the search engines
If more dofollow backlinks pointing back to your website there may be the chance of getting higher rank in searchengine.

Backlinks play very important role in getting higher PR. Google counts all backlinks which are dofollow to count importance of a particular site. Like this comments on forum help to increase backlinks as well as increase site's popularity.